CHAPTER 13

The Optic Nerve

Anatomy and Physiology

The optic nerve is a central nervous system (CNS) fiber pathway connecting the retina and the brain. The peripheral receptors, retinal rods and cones, are stimulated by light rays that pass through the cornea, lens, and vitreous. They send impulses to the inner nuclear or bipolar layer; cells there send axons to the ganglion cell layer (Figure 13.1). There are nearly 1.2 million ganglion cells and their axons that make up the optic nerve. The photoreceptor layer is the deepest layer of the retina; it lies adjacent to the choroid, and light must pass through the more superficial layers to reach it. The rods, more numerous than the cones, are scattered diffusely throughout the retina but are absent in the macula. They respond to low-intensity stimulation and mediate night vision, peripheral vision, and perception of movement. They cannot perceive color. Cones are also present throughout the retina but are concentrated in the macula lutea. The macula consists entirely of cones; it is the point of central fixation and the site of greatest visual acuity and color perception. The macular cones have a 2:1 ratio with ganglion cells, the highest in the eye. The macula (L. “spot”) is a small shallow depression in the retina that lies temporal to the disk (Figure 13.2). It has a slightly different color than the surrounding retina that can be seen with the ophthalmoscope. The fovea (L. “pit”) centralis is a tiny depression that lies in the center of the macula. The foveola is an even tinier depression in the center of the fovea. It is the point of most acute vision because the overlying retinal layers are pushed aside and light falls directly on the receptors; the foveola is the optical center of the eye. The macula is responsible for the central 15 degrees of vision and the discrimination of colors and fine visual details; its cones are stimulated by light of relatively high intensity and colors. The optic disk, or papilla, is the ophthalmoscopically visible tip of the intraocular portion of the optic nerve. The nerve head is a 1.5 × 1.8 mm vertical ellipse, and it appears as a pink to yellowish-white disk. The disk normally inserts into the retina perpendicularly. When the angle is less than 90 degrees, a rim or crescent of choroid or sclera appears on the temporal side and the nasal side may appear elevated (tilted disk). It contains no receptor cells, does not respond to visual stimuli, and is responsible for the physiologic blind spot. The macula, not the disk, forms the center of the retina, and the macular fixation point is the center of the clinical visual field (VF).

FIGURE 13.1 The layers of the retina and their relationship to the optic nerve. The inner limiting membrane is the most superficial structure; light must pass through the other layers to reach the rod and cone layer. (Modified from Ramon y Cajal S. Histologie du Système Nerveux de l’Homme et Des Vertebres, vol 2. Paris: A. Maloine, 1909, 1911.)

FIGURE 13.2 Structure of the eyeball.

The retinal ganglion cell axons form the retinal nerve fiber layer (NFL) as they stream toward the disk to exit through the lamina cribrosa (L. “sieve”), the collagenous support of the optic disk. Loss of axons and other abnormalities involving the NFL can sometimes be appreciated ophthalmoscopically. Using the red-free light of the ophthalmoscope helps visualize the NFL. Myelin in the optic nerve is CNS myelin, formed by oligodendroglia. The axons are unmyelinated in the retina and on the papillary surface but become myelinated at the posterior end of the optic nerve head as they pass through one of 200 to 300 holes in the lamina cribrosa. In about 1% of individuals, myelin extends into the peripapillary retinal NFL (myelinated nerve fibers). Optic nerve axons primarily carry visual impulses, but they also transmit the impulses that mediate accommodation and reflex responses to light and other stimuli. Optic nerve signals are coded spatially because of the location of cells in the retina, and they are also coded temporally because the frequency and pattern of firing relays information.

Macular vision is a critical function, and the projection of the macula to the optic nerve is massive. There are approximately 1.2 million fibers in each optic nerve; about 90% arise from the macula. Because of this preponderance of macular fibers, early signs of optic nerve disease reflect macular function: impaired color vision, impaired acuity, and central scotoma. A dense collection of axons, the papillomacular bundle (PMB), travels from the nasal hemimacula to enter the temporal aspect of the disk (Figure 13.3). Fibers from the temporal hemiretina and hemimacula arch around the macula and enter the disk as the superior and inferior retinal arcades. Lesions involving these arcades may create arcuate VF defects that have an arching shape. The horizontal temporal raphe demarcates superiorly from inferiorly sweeping axons traveling from the temporal hemimacula to the disk. All of the axons from the macula gather into the PMB as it enters the optic nerve. The fibers of the PMB are very vulnerable to toxins, ischemia, and pressure.

FIGURE 13.3 The optic disk and associated structures. Axons destined to form the bulk of the fibers in the optic nerve arise from the macula, those from the nasal side form the papillomacular bundle, and those from the temporal hemimacula enter the disk as superior and inferior arcades. Spontaneous venous pulsations are best seen by looking at the tip of the column of one of the large veins on the disk surface.

The organization of the visual afferent system is not random. Tight retinotopic correlation prevails throughout the system; each point on the retina has a specific representation in the optic nerve, the chiasm, the tract, the radiations, and the cortex. The PMB, which forms the bulk of optic nerve axons, runs as a discrete bundle inside the optic nerve. The VF maintains its basic shape and structure throughout the system, although its orientation within the visual pathways changes (Figure 13.4). Fibers from the temporal hemiretina are located in the temporal half of the optic nerve, whereas fibers from the nasal hemiretina are located medially. Upper retinal fibers are located superiorly and lower retinal fibers inferiorly in the optic nerve; this relationship is retained except in the optic tract and lateral geniculate body (LGB).

FIGURE 13.4 The grouping of visual fibers from the retinal quadrants and macular area in the optic nerve, optic tract, lateral geniculate body (LGB), and occipital cortex.

The optic nerve extends from the retina to the optic chiasm; it is approximately 5 cm long. It is conventionally divided into four portions: intraocular (1 mm; the disk), intraorbital (about 25 mm), intracanalicular (about 9 mm), and intracranial (12 to 16 mm). The nerve is organized into 400 to 600 fascicles separated by connective tissue septa. The intraorbital portion is surrounded by fat (Figure 13.5).

FIGURE 13.5 Optic nerve exposed from above with fat and the roof and lateral wall removed. The intraocular segment (a) is within the globe. The intraorbital segment (b) runs through the orbit to the entrance of the optic canal depicted by the left-most blue dot. The short intracanalicular segment (c) courses between the two blue dots. The intracranial segment (d) continues to its junction with the optic chiasm (blue bar). (Courtesy Dr. John B. Selhorst.)

The intracranial dura is continuous with the investments of the optic nerve; at the posterior globe, the dura fuses with Tenon’s capsule, and at the optic foramen, it is adherent to the periosteum. The pia and arachnoid also continue from the brain and envelop the optic nerve. They fuse with the sclera where the nerve terminates. The intracranial meninges extend forward along the optic nerves for a variable distance, forming the vaginal sheaths (Figure 13.2). Through these sheaths, the intracranial subarachnoid space continues along the nerves and may transmit increased intracranial pressure, causing papilledema. Variations in vaginal sheath anatomy may explain the occasional asymmetry of papilledema. Decompression of the optic nerves by opening the sheaths is sometimes done to treat papilledema that threatens vision. The intervaginal space lies between the dura and the pia, divided by the arachnoid into a small subdural and a larger subarachnoid space. The ophthalmic artery, the ciliary ganglion and nerves, and the nerves to the extraocular muscles lie close to the optic nerve in the orbital apex. The intraorbital optic nerve is sinuous, with about 8 mm of redundant length to accommodate eye movement. This excess of length allows about 9 mm of proptosis before the nerve begins to tether.

In the peripheral portion of the nerve, near the eye, the PMB is positioned laterally and slightly inferiorly; this separates the temporal fibers into dorsal and ventral quadrants. These in turn crowd and somewhat displace the nasal quadrants (Figure 13.4). As the nerve approaches the chiasm, the PMB moves toward its center.

The intracanalicular portion of the optic nerve begins as it traverses the optic foramen at the orbital apex. The orbital opening of the canal is a vertical ellipse; the intracranial end is a horizontal ellipse. The intracanalicular portion is fixed tightly inside the optic canal with little room to move; intracanalicular lesions can compress the optic nerve while they are still small and difficult to visualize on imaging studies (the “impossible meningioma”). The ophthalmic artery and some filaments of the sympathetic carotid plexus accompany the nerve through the canal.

After traversing the orbit and optic canal, the two optic nerves exit from the optic canals and rise at an angle of about 45 degrees to unite at the optic chiasm, so named because of its resemblance to the Greek letter chi (χ) (Figure 13.6). The orbital surface of the frontal lobes lies just above the intracranial optic nerves. The chiasm typically lies about 10 mm above the pituitary gland, separated by the suprasellar cistern. Fibers from the temporal retina continue directly back to enter the ipsilateral optic tract. Fibers from the nasal retina decussate to enter the opposite optic tract.

FIGURE 13.6 The macular fibers decussate as a separate compact bundle, inferior retinal (superior visual field [VF]) fibers cross inferiorly, and superior retinal (inferior VF) fibers cross superiorly. Masses impinging from below (e.g., pituitary adenoma) tend to cause early defects in the superior temporal fields; masses impinging from above (e.g., craniopharyngioma) tend to cause early defects in the inferior temporal fields.

In 80% of the population, the chiasm rests directly above the sella. In 10%, the chiasm sits forward over the tuberculum sellae with short optic nerves and long optic tracts (prefixed); in the other 10%, the chiasm sits posteriorly over the dorsum sellae with long optic nerves and short optic tracts (postfixed) (Figure 13.7). The position of the chiasm in relation to the sella and the neoplasia-prone pituitary gland influences the clinical presentation of masses in the region.

FIGURE 13.7 The normal position of the chiasm is shown in the top drawing. When the chiasm is prefixed, the optic nerves are short, the chiasm sits forward over the sella, and the optic tracts are long. When the chiasm is postfixed, the optic nerves are long, the chiasm sits posteriorly over the sella, and the optic tracts are short.

The basic scheme of the chiasm with temporal hemiretinal fibers continuing ipsilaterally and nasal hemiretinal fibers decussating is straightforward (Figure 13.8). But there are intricacies in the chiasmal crossing. In the process of decussating, fibers from the inferior nasal quadrant loop forward into the opposite optic nerve for a short distance before turning back again, forming Wilbrand’s knee (Figure 13.9, see junctional scotoma in “Scotomas” section). In addition, some of the upper nasal fibers loop back briefly into the ipsilateral optic tract before decussation. In the chiasm, the fibers from the upper retinal quadrants lie superior and those from the lower quadrants inferior (Figure 13.6). Inferior nasal fibers decussate anteriorly and inferiorly in the chiasm, whereas superior nasal fibers cross posteriorly and superiorly, accounting for the difference in the pattern of evolution of the field defect in infrachiasmatic versus suprachiasmatic lesions (Figure 13.9). Macular fibers more or less decussate as a group, forming a miniature chiasm within the chiasm, primarily in the posterior superior portion.

FIGURE 13.8 The course of the visual fibers from the retina to the occipital cortex. A to G show the sites of various lesions that may affect the fields of vision.

FIGURE 13.9 A mass impinging on the optic nerve at its junction with the chiasm, producing a junctional scotoma.

The cavernous sinuses and carotid siphons lie just lateral to the chiasm on either side. The anterior cerebral and anterior communicating arteries are in front and above, and the third ventricle and hypothalamus are behind and above. The sella turcica and sphenoid sinus lie below. The circle of Willis lies above, sending numerous small perforators to supply the chiasm. The ophthalmic artery runs alongside the optic nerve within the same dural sheath through the canal and orbit. About 8 to 12 mm posterior to the globe, the artery enters the nerve and runs along its center to the optic disk, where it becomes the central retinal artery, which pierces the nerve and runs forward onto the disk. The central retinal artery divides at the disk head into superior and inferior branches, which supply the retina. Other terminal branches of the ophthalmic, the short posterior ciliary arteries and choroidal vessels, form an arterial network, the circle of Zinn-Haller, which supplies the disk; the central retinal artery makes only a minimal contribution to the vascular supply of the optic disk.

Posterior to the optic chiasm, the uncrossed fibers from the ipsilateral temporal hemiretina and the crossed fibers from the contralateral nasal hemiretina form the optic tract. About 55% of the axons of the optic tract arise from the contralateral nasal retina and 45% from the ipsilateral temporal retina, which roughly corresponds to the ratio of the area of the temporal field to the nasal field. The tracts contain approximately 80% visual afferents and 20% pupillary afferents. The tracts extend from the chiasm to the LGB, where the majority of fibers terminate. Retinotopic organization is maintained in the optic tract, but the orientation changes. There is a gradual inward rotation, so fibers from the upper retina assume a medial position, whereas those from the inferior retina lie lateral. Fibers of the PMB gradually assume a dorsal and lateral position, wedged between the upper and lower retinal fibers (Figure 13.4). The retinotopic organization in optic tracts is not as precise as elsewhere, which may contribute to the incongruity of VF defects that are characteristic of optic tract lesions.

Afferent fibers from the pupil leave the tract just anterior to the geniculate to enter the pretectal area of the midbrain (Figure 13.10). The visual afferents synapse in the geniculate on second-order neurons, which give rise to the geniculocalcarine pathway (optic radiations).

FIGURE 13.10 Pupillary afferent fibers from the right eye are crossed and uncrossed and run in both optic tracts. They leave the tract before the LGB and send projections to the pretectal region bilaterally. The Edinger-Westphal nucleus sends pupillomotor fibers through the third cranial nerve to the ciliary ganglion, and postganglionic fibers innervate the pupil sphincter. Because of the bilaterality of the pathways, a light stimulus in the right eye causes pupillary constriction in both eyes.

There are six neuronal layers in the LGB, separated by myelinated nerve fibers. Uncrossed fibers from the ipsilateral temporal hemiretina synapse in layers 2, and 5; those from the contralateral nasal hemiretina synapse in layers 1, and 6. Upper retinal fibers remain medial and lower ones lateral (Figure 13.4). Macular fibers occupy an intermediate position in the dorsal, middle, and somewhat caudal portion. The LGB has large magnocellular and small parvocellular neurons. Some of the visual fibers pass over or through the LGB to terminate in the pulvinar of the thalamus, but the significance of this connection has yet to be determined for vision or visual reflexes. The magnocellular projections seem to process movement and depth, whereas the parvocellular projections mediate shape, pattern, and color.

The axons of LGB neurons pass posteriorly to form the geniculocalcarine tract, or optic radiations, and terminate in the calcarine cortex of the occipital lobe (Figure 13.11). Leaving the LGB, the optic radiations pass through the retrolenticular portion of the internal capsule and then fan out. Retinotopically, upper retinal fibers resume an upper, and lower retinal fibers a lower, position in the radiations, with fibers subserving central vision intermediate between the two other bundles. Inferior retinal fibers arch anteriorly into the temporal lobe, sweeping forward and laterally above the inferior horn of the ventricle to run within 5 to 7 cm of the temporal tip, then laterally, down, and backward around the inferior horn. This creates a great arching shape referred to as Meyer’s loop (loop of Meyer and Archambault). The inferior retinal fibers then course through the temporal and occipital lobes. Peripheral retinal fibers loop further forward than macular fibers. Fibers from the superior retina run directly back in the deep parietal lobe in the external sagittal stratum, lateral to the posterior horn of the lateral ventricle. The inferior, or ventral, radiations may mediate recognition of visual objects, whereas the dorsal pathway processes spatial information and recognition of movement.

FIGURE 13.11 The course of the geniculocalcarine fibers. A. Medial view.B. Inferior view.

Approaching the occipital lobe, fibers from the upper and lower retina again converge. The primary visual cortex (calcarine area or striate cortex) lies in Brodmann’s area 17 on the medial surface of the occipital lobe. Lower retinal fibers terminate on the lower lip of the calcarine fissure (lingual gyrus) and upper retinal fibers on the upper lip of the calcarine fissure (cuneus). Macular fibers are first lateral and then form the intermediate portion of the geniculocalcarine pathway, continuing to the posterior pole of the occipital lobe. The divergence and convergence of fibers throughout the visual pathway influences the shape and congruity of VF defects, which have localizing value.

Fibers that carry visual impulses from the peripheral portions of the retina terminate on the anterior third or half of the visual cortex of the occipital lobe in concentric zones; macular fibers terminate in the posterior portion (Figure 13.12). The most peripheral parts of the retina are represented most anteriorly in the calcarine cortex; the closer a retinal point lies to the macula, the more posterior its calcarine representation. This culminates in the representation of the macula at the occipital pole. The nasal hemiretina representation extends farther forward than the temporal (the temporal field is more extensive than the nasal), creating a portion of retina for which no homology exits in the opposite eye. This unpaired nasal retina is represented in the most anterior portion of the calcarine cortex, near the area of the tentorium, just outside the binocular VF, which creates an isolated temporal crescent in each VF. Sparing or selective involvement of this monocular temporal crescent has localizing value. The macula has a wider cortical distribution in the striate cortex than in the peripheral retina. It is represented in a wedge-shaped area with its apex anterior. The central 10 to 15 degrees of the VF occupy 50% to 60% of the visual cortex.

FIGURE 13.12 Fibers from the macula synapse in the geniculate and then project to the occipital tip. The most peripherally located retinal ganglion cells synapse in the geniculate and then loop far forward in Meyer’s loop before terminating in the most anterior portion of the calcarine cortex. The most anterior and medial portions of the cortex receive projections from the monocular temporal crescent, which represents the nasal portion of the retina that extends far forward and is the most peripheral part of the retina.

To summarize the retinotopic organization of the visual system, upper retinal fibers remain upper and lower fibers lower throughout except in the tract and LGB where upper becomes medial and lower becomes lateral. The corresponding VF abnormalities can be deduced.

The striate cortex is the sensory visual cortex. It receives afferents via the myelinated stripe or line of Gennari, because of the abundant myelinated fibers in the fourth layer of the calcarine cortex, which gives this area its distinctive appearance and name. Its physiology is complex. Neurons are arranged in parallel, vertically oriented, ocular dominance columns and complex units called hypercolumns. One hypercolumn can process information from a focal region of the VF. There may be interhemispheric connections through the corpus callosum to synchronize information generated from the two sides. Surrounding the striate cortex are the visual association areas. Area 18, the parastriate or parareceptive cortex, receives and interprets impulses from area 17. Area 19, the peristriate or perireceptive cortex, has connections with areas 17 and 18 and with other portions of the cortex. It functions in more complex visual recognition, perception, revisualization, visual association, size and shape discrimination, color vision, and spatial orientation.

The anterior choroidal artery from the internal carotid and thalamoperforators from the posterior cerebral supply the optic tract. The geniculate is perfused by the anterior choroidal and thalamogeniculate branches from the posterior cerebral. Perhaps because of this redundant blood supply, vascular disease only rarely affects the optic tract or lateral geniculate. Meyer’s loop receives blood supply primarily from the inferior division of the middle cerebral artery, whereas the optic radiations in the parietal lobe are perfused via the superior division. The occipital lobe is supplied primarily by the posterior cerebral artery. Collaterals from the anterior and middle cerebral may provide additional perfusion to the macular areas at the occipital tip. The parietal smooth pursuit optomotor center and its projections are supplied by the middle cerebral.

Optic Reflexes

Fibers subserving the pupillary light reflex and other optic reflexes pass through the pregeniculate pathways in the same fashion as fibers subserving vision. They leave the optic tract just before it reaches the LGB. Pupillary light reflex fibers travel to the pretectal nuclei, just rostral to the superior colliculus; from the pretectum, axons are sent to synapse on the Edinger-Westphal nuclei. Some light reflex fibers project to the ipsilateral pretectal nucleus to mediate the direct light reflex; others decussate through the posterior commissure to mediate the consensual light reflex (Figures 13.8 and 13.10). Parasympathetic fibers from the Edinger-Westphal nuclei are carried by the oculomotor nerve to the pupillary sphincter.

Fibers controlling somatic visual reflexes, such as turning of the head and eyes toward a visual stimulus, synapse in the superior colliculus. From there tectospinal tract fibers descend to more caudal brainstem nuclei to execute the reflex response. The internal corticotectal tract is made up of fibers that run from areas 18 and 19 of the occipital cortex to the superior colliculus to subserve reflex reactions through connections with the eye muscle nuclei and other structures. Fibers that carry impulses having to do with visual-palpebral reflexes (such as blinking in response to light) go to the facial nuclei.

Clinical Examination and Disorders of Function

Optic nerve function is tested by examining the various modalities of vision: the visual acuity, the VFs, and special components of vision, such as color vision and day and night vision. The optic nerve is the one cranial nerve that can be visualized directly, and no neurologic, or indeed general, physical examination is complete without an ophthalmoscopic inspection of the optic disk and the retina.

Before performing the optic nerve examination, look for local ocular abnormalities such as conjunctival irritation, corneal scarring or opacity, foreign bodies, photophobia, or an ocular prosthesis. The presence of a unilateral arcus senilis with ipsilateral carotid disease has been reported. In Wilson’s disease (hepatolenticular degeneration), a yellowish-orange brown coloration 1 to 3 mm wide (Kayser-Fleischer ring) may be seen around the rim of the cornea, more easily in light-eyed individuals (Chapter 30). It is due to copper deposition in the posterior stroma and in Descemet’s membrane and best seen with a slit lamp. Cataracts may be present in patients with myotonic dystrophy, certain rare hereditary conditions with disturbed lipid or amino acid metabolism, and in many other conditions. Lisch nodules are pigmented iris hamartomas that are highly suggestive of NF1 (Figure 13.13). Proptosis, chemosis, and tortuous (“corkscrew”) blood vessels in the conjunctiva occur with carotid cavernous fistula (Chapter 21). Other causes of unilateral proptosis include thyroid eye disease, meningocele, encephalocele, and histiocytosis X. Other potentially relevant findings might include jaundice, evidence of iritis, dysmorphic changes (e.g., epicanthal folds), xanthelasma due to hypercholesterolemia, corneal clouding from mucopolysaccharidosis, keratoconjunctivitis sicca due to Sjögren’s syndrome or other collagen vascular diseases, ocular complications of upper facial paralysis, depositions of amyloid in the conjunctiva, pigmented pingueculae due to Gaucher’s disease, tortuous conjunctival vessels in ataxia telangiectasia, scleritis in Wegener’s granulomatosis, lens dislocation in Marfan’s syndrome, homocystinuria or Ehlers-Danlos syndrome, and nonsyphilitic interstitial keratitis in Cogan’s syndrome. Hypertelorism can be seen in a number of neurologic conditions. Blue sclera can occur in Ehlers-Danlos syndrome, osteogenesis imperfecta, and occasionally in Marfan’s syndrome. Basal skull fractures often cause bilateral periorbital ecchymosis (raccoon eyes).

FIGURE 13.13 Lisch nodules are elevated, pale brown lesions that vary in appearance depending on the underlying color of the iris. The prevalence in patients with NF1 increases from birth to about 50% of 5-year-olds% of 15-year-olds, and 95% to 100% of adults over the age of 30. (Reprinted from Gerstenblith AT, Rabinowitz MP. The Wills Eye Manual: Office and Emergency Room Diagnosis and Treatment of Eye Disease. 6th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2012, with permission.)

Visual Acuity

Ideally, the eyes are examined individually. When testing acuity and color vision, it is important to occlude the eye not being tested. Visual acuity is a measure of the eye’s ability to resolve details; it depends on several functions. The intensity threshold reflects the sensitivity of the retina to light; the minimum visibility is the smallest area that can be perceived, and the minimum separability is the ability to recognize the separateness of two close points or lines. Visual acuity charts, such as the Snellen chart for distance and the near card for near, consist of letters, numbers, or figures that get progressively smaller and can be read at distances from 10 to 200 ft by normal individuals (Figure 13.14). Snellen charts have certain limitations, the most critical is the nonlinear variation in the sizes of the letters from line to line, and the Early Treatment in Diabetic Retinopathy Study (ETDRS) charts have become increasingly popular.

FIGURE 13.14 Snellen test chart.

The difference between near and distance vision and between vision with and without correction are points of primarily ophthalmologic interest. For neurologic purposes, only the patient’s best-corrected visual acuity is pertinent. Near acuity is not as accurate as distant, especially if the card is not held at the required 14 in. Refractive errors, media opacities, and similar optometric problems are irrelevant. Acuity is always measured using the patient’s accustomed correction. Ophthalmologists and neuro-ophthalmologists often employ more detailed methods (e.g., full refraction) to clarify the refractive component of a patient’s visual impairment. In infants and children, acuity can be estimated by blink to threat or bright light, following movements, and the pupillary reactions. At the age of 4 months, acuity may be 20/400; it gradually increases, reaching normal levels at about age 5.

For distance vision measurement in the United States, a Snellen chart or similar is placed 20 ft from the patient; at that distance, there is relaxation of accommodation, and the light rays are nearly parallel. The eyes are tested separately, and by convention, the right eye is tested first. In countries using the metric system, the distance is usually given as 6 m. The ability to resolve test characters (optotypes) approximately 1-in high at 20 ft is normal (20/20 or 6/6) visual acuity. These characters subtend 5 minutes of visual arc at the eye; the components of the characters (e.g., the crossbar on the A) subtend 1 minute of arc. The acuity is the line where more than half of the characters are accurately read. If the patient can read the 20/30 line and two characters on the 20/25 line, the notation is 20/30 + 2. Two mistakes or two extra letters are allowed per line. By conventional notation, the distance from the test chart or 6, is the numerator, and the distance at which the smallest type read by the patient should be seen by a person with normal acuity is the denominator. An acuity of 20/40 (6/12) means the individual must move in to 20 ft to read letters a normal person can read at 40 ft. This does not mean the patient’s acuity is one-half of normal. In fact, an individual with a distance acuity of 20/40 has only a 16.4% loss of vision.

Because few neurology clinics, offices, or hospital rooms have 20-ft eye lanes, testing is commonly done at a closer distance. Neurologists frequently assess vision with a near card. Though examination of distance vision is preferable, the requisite devices are generally not at hand. There are pocket cards designed for testing at 6 ft, a convenient distance that usually eliminates the need for presbyopic correction. Near vision is tested with a near card, such as the Rosenbaum pocket vision screening card, held at the near point (14 in or 35.5 cm). Jaeger reading cards are still used occasionally (Box 13.1). Good lighting is essential. A penlight shone directly on the line being read is useful for bedside testing.

BOX 13.1

Jaeger Notation

Jaeger’s test types are ordinary printer’s types, graded from fine (Jaeger 0) to coarse, also used for near testing. The physical optics of the Jaeger system are crude. The numbers refer to the boxes in the Austrian print shop from which Jaeger selected the type in 1854. Jaeger 0 corresponds approximately to an acuity of 20/20. As a rough approximation of near vision, the examiner may use different sizes of ordinary print. Newspaper want-ad text is approximately J-0, regular newsprint J-6, and newspaper headlines J-17.

If the patient cannot read the 20/200 line at 20 ft, the distance may be shortened and the fraction adjusted. Ability to read the line at 5 ft is vision of 5/200, equivalent to 20/800. Vision worse than the measurable 20/800 is described as count fingers (CFs), hand motion (HM), light perception (LP), or no light perception (NLP). The average finger is approximately the same size as the 20/200 character, so ability to count fingers at 5 ft is equivalent to an acuity of 20/800.

When a patient has impaired vision, an attempt should be made to exclude refractive error by any available means. If the patient has corrective lenses, they should be worn. In the absence of correction, improvement of vision by looking through a pinhole suggests impairment related to a refractive error. Commercial multi-pinhole devices are available. A substitute can be made by making three or four holes with a pin in a 3 × 5 card in a circle about the size of a quarter. The multiple pinholes help the patient locate one. The patient should then attempt to read further down the acuity card through the pinhole. The pinhole permits only central light rays to enter the eye. These are less likely to be disrupted by refractive errors such as presbyopia and astigmatism. If a pinhole was used, make some notation, such as 20/20 (ph). If the visual impairment is due to a neurologic process, such as optic neuritis (ON), vision will not improve with a pinhole. Under some circumstances, such as with opacities in the media (e.g., cataract), vision may get worse with pinhole.

Other ocular causes of reduced visual acuity include a macular lesion, media opacity such as cataract or vitreous hemorrhage, and corneal opacities or irregularities. Neurologic processes that affect the optic nerve or chiasm may cause impaired acuity. Retrochiasmal lesions affect visual acuity only if they are bilateral. Suspected functional visual loss because of hysteria or malingering is best evaluated by an ophthalmologist, who has the proper tools to answer the question. Clever and determined patients with functional visual loss present a major challenge. There may be certain clues (Box 13.2).

BOX 13.2

Nonorganic (Functional) Visual Loss

A truly blind person can sign his name without difficulty. A functionally blind patient often cannot. A truly blind person asked to look at his hand will look wherever proprioception tells him his hand should be; a functionally blind person may gaze in any direction and perhaps never where the hand actually is (Schmidt-Rimpler test). A truly blind person can touch his forefingers together without difficulty; a functionally blind person may make half-hearted inaccurate thrusts. The presence of normal visual, menace, fixation, and emergency light reflexes (see Chapter 16) excludes organic blindness. A functionally blind person ignorant of the laws of reflection may have much improved vision reading the image of an acuity chart held to his chest in a mirror 10 ft away compared to reading the actual chart at 20 ft; the acuity in fact should be the same. Some patients with functional blindness can suppress optokinetic nystagmus (OKN) responses and the visual evoked response (VER). An excellent test is to have the patient look into a large mirror that can be held and moved. Tilting and moving the mirror will elicit OKN responses because the entire visual environment is moving. The patient cannot suppress or “blur out” by willfully failing to fixate on a single target, as he may be able to do with OKN or VER.

The term amblyopia refers to impaired vision because of an organic process in the absence of a demonstrable lesion. The mechanism is poorly understood. Suppression amblyopia is the visual impairment in one eye because of preferential use of the opposite eye in a patient with congenital strabismus. Suppression amblyopia is also referred to as amblyopia ex anopsia (amblyopia from disuse). Many other varieties of amblyopia have been described, including alcoholic, toxic, traumatic, and uremic amblyopia. Amaurosis means blindness of any type, but in general usage, it means blindness without primary eye disease or loss of vision secondary to disease of the optic nerve or brain.

Color Vision; Day and Night Vision

Color blindness (achromatopsia) is an X-linked condition present in about 3% to 4% of males. The most common hereditary dyschromatopsia is an X-linked red-green defect. Disturbances of color vision may also occur in neurologic conditions. Loss of color vision may precede other visual deficits. Color deficits may be partial or total. Color plates or pseudoisochromatic plates (Ishihara, Hardy-Ritter-Rand [HRR], or similar) formally and quantitatively assess color vision. Pseudoisochromatic plates were originally designed to screen for congenital dyschromatopsias. The HRR plates, which contain blue and purple figures that screen for tritan defects, may be more helpful in detecting acquired dyschromatopsia because of some optic neuropathies. Having the patient identify the colors in a fabric, such as a tie or a dress, can provide a crude estimate of color vision.

Acquired dyschromatopsia may result from macular, retinal, optic nerve, chiasmal, or retrochiasmal lesions. Monocular loss of acuity, deficits in color vision, and an afferent pupillary defect (APD) are highly characteristic of an ipsilateral optic neuropathy. Acquired optic nerve diseases usually cause a red/green color deficiency, but there are several exceptions, such as glaucoma and dominant optic atrophy. Impaired color vision with only mildly reduced acuity is suggestive of optic neuropathy; color deficits associated with more severe acuity loss suggest maculopathy.

Fading or bleaching of colors is a real but uncommon complaint in optic nerve disease. Red perception is usually lost first. Desaturation to red, or red washout, describes a graying down or loss of intensity of red. The bright red cap on a bottle of mydriatic drops is a common test object. The patient compares the brightness or redness in right versus left hemifields, temporal versus nasal hemifields, or central versus peripheral fields. No right/left or temporal/nasal desaturation to red occurs normally. Red does normally look brighter in the center of the VF than off center; reversal of this pattern suggests impairment of central vision. The normal red appears washed out, or changes to pink to orange to yellow to colorless as color perception is lost. Because optic neuropathies affect macular fibers, patients lose the ability to read pseudoisochromatic plates. The flight of colors phenomenon is the series of color perceptions that follows shining a bright light into the eye. With impaired color vision, the flight of colors may be reduced or absent. Patients may also compare the brightness or intensity of an examining light in one eye versus the other. A diminution of brightness on one side suggests optic nerve dysfunction; it is sometimes referred to as a subjective APD, relative APD, or Marcus-Gunn pupil. Its significance is the same as for red desaturation. The APD is discussed in more detail in Chapter 14.

Day blindness (hemeralopia) is a condition in which vision is better in dim lighting than in bright. It occurs in various conditions causing a central scotoma, in early cataracts; it is a rare side effect of trimethadione. Night blindness (nyctalopia) is much poorer vision in feeble illumination than occurs normally. It is common in retinitis pigmentosa and can occur in chronic alcoholism, Leber’s hereditary optic neuropathy (LHON), and xerophthalmia due to vitamin A deficiency.

The Visual Fields

The VF examination is a very important and, unfortunately, often omitted part of the neurologic examination. The VF is the limit of peripheral vision, the area in which an object can be seen while the eye remains fixed. Macular vision is sharp. Peripheral images are not as distinct, and objects are more visible if they are moving. The normal VF extends to 90 to 100 degrees temporally, about 60 degrees nasally to 60 degrees superiorly, and 60 to 75 degrees inferiorly. The field is wider in the inferior and temporal quadrants than in the superior and nasal quadrants (Figure 13.15). There are individual variations in the field of vision, dependent to some extent on the facial configuration, the shape of the orbit, the position of the eye in the orbit, the width of the palpebral fissure, and the amount of brow projection or the size of the nose. However, these changes are seldom clinically relevant. With binocular vision, the VFs of the two eyes overlap except for the unpaired temporal crescent extending from 60 to 90 degrees on the horizontal meridian, which is seen by one eye only. The monocular temporal crescent exists because of the anatomy of the retina. The nasal retina extends farther forward, more peripherally, than the temporal. This is the true reason that the temporal VF is more expansive, not because the nose is blocking the nasal field.

FIGURE 13.15 The normal VFs.

VF examination results are most accurate in an individual who is alert and cooperative and will maintain fixation. Wandering of the eye impairs the evaluation. Crude assessment is possible even in uncooperative patients if the target is interesting enough (e.g., food or paper money). Fatigue and weakness may lengthen the latency between perception of the test object and the response to it, giving a false impression of VF deficit. Close cooperation, good fixation, and adequate illumination are essential for mapping of the blind spot and delineation of scotomas.

Clinicians use several different methods for VF evaluation. The time and energy expended on bedside confrontation testing depends on the patient’s history and on the facilities available for formal field testing with tangent (Bjerrum) screen (central 30 degrees) or perimetry (entire field). Even sophisticated confrontation testing cannot approach the accuracy of formal fields.

The confrontation VF exam can be tailored to the circumstances and done as superficially or as thoroughly as the situation requires. Sophisticated bedside techniques can explore the VFs in detail if circumstances warrant. If the patient has no specific visual complaint, and if other aspects of the history and examination do not suggest a field defect is likely, then a screening exam is appropriate. This can be accomplished rapidly and with great sensitivity using small amplitude finger movements in the far periphery of the VF. Recall that the VFs extend temporally to 90+ degrees. Extending elbows and index fingers, the examiner should position the fingers nearly directly lateral to the lateral canthus at a distance of about 24 in. Superficially, this appears to be a binocular examination, but, properly placed, the finger targets are actually in the unpaired monocular temporal crescent part of the VF. With the targets positioned, make a small amplitude flexion movement with the tip of one index finger, perhaps 2 cm in amplitude. Have the patient “point to the finger that moves.” This language is more efficient than attempting a right-left verbal description where the patient’s and examiner’s rights and lefts are reversed. Stimuli should be delivered in each upper quadrant individually, then both together, and then similarly for the lower quadrants. Including bilateral simultaneous stimuli is necessary to detect subtle defects, which may be manifested only by extinction of one stimulus on double simultaneous stimulation. This technique of small finger movements in the far periphery in both upper and lower quadrants is an excellent screen; when properly done, even binocularly, this technique misses few VF defects. Large amplitude finger wiggles near the center are insensitive. Always bear in mind that primary ophthalmologic disorders such as glaucoma, diabetic retinopathy, and retinal detachment can also alter the VFs.

With any hint of abnormality, or if the patient has or could be expected to have a visual problem, higher-level testing is in order. Examining monocularly, techniques include having the patient assess the brightness and clarity of the examiner’s hands as they are held in the right and left hemifields, in both upper and lower quadrants, or having the patient count fingers fleetingly presented in various parts of the field. Because of the over-representation of central vision in the CNS, assessing each quadrant within the central 10 to 20 degrees is important.

More exacting techniques compare the patient’s field dimensions with the examiner’s, using various targets—still or moving fingers, the head of a cotton swab, colored pinheads, or similar objects. Impairment of color perception also occurs with lesions of the posterior visual pathways. Loss of VF to testing with a red object may be apparent even when the fields are intact to a white object. Positioning the patient and examiner at the same eye level, and gazing eyeball to eyeball over an 18- to 24-in span, targets introduced midway between and brought into the VF along various meridians should appear to both people simultaneously in all parts of the field except temporally, where the examiner must simply develop a feel for the extent of a normal field (Figure 13.16). Even in expert hands, confrontation fields are relatively gross and more precision required perimetry.

FIGURE 13.16 Confrontation method of testing the VFs.

For obtunded, uncooperative, or aphasic patients, paper money (the larger the denomination the better) makes a compelling target. Even if the examiner has only a $1 bill, suggest to the patient that it might be $100. The patient who can see will glance at or reach for the object. Children may respond to keys (no jingling), candy, or other visually interesting objects. Infants may turn the head and eyes toward a diffuse light within a few days after birth. Moving a penlight into the VF and noting when the patient blinks is sometimes useful. Checking for blink to threat—the menace reflex—provides a crude last resort method. The examiner’s hand or fingers are brought in rapidly from the side, as if to strike the patient or poke him in the eye. The patient may wince, drawback, or blink. The threatening movement should be deliberate enough to avoid stimulating the cornea with an induced air current.

Testing central fields can include having the patient gaze at the examiner’s face and report any defects, such as a missing or blurred nose. Having the patient survey a grid work (Amsler grid, graph paper, or a quickly sketched homemade version) while fixing on a central point is a sensitive method to detect scotomas (Figure 13.17). Probing the central field with a small white or red object may detect moderate or large scotomas. With a cooperative patient, one can estimate the size of the blind spot. Amsler grid testing is helpful in detecting central and paracentral scotomas. Small deficits suggest macular disease and may be missed on perimetry.

FIGURE 13.17 The Amsler grid for testing the central VFs. (1) Test vision with one eye at a time, and use normal glasses for reading. (2) Hold chart at normal reading distance. (3) Stare at central dot and look for distortion or blind spots in the grid.

Pandit et al. compared the sensitivity of seven confrontation VF examining methods in patients whose formal fields showed small or shallow defects. The most sensitive method was examining the central VF with a 5-mm red target; the next most sensitive was comparing red color intensity. These together had a sensitivity of 76%. Description of the examiner’s face and quadrant finger counting were the least sensitive. All of the confrontation methods had high specificity. In a similar study, Kerr et al. compared seven common confrontation VF tests to Humphrey VF in 301 eyes in patients recruited from a neuro-ophthalmology clinic, and therefore at high risk for having a VF defect. Anterior visual pathway lesions accounted for 78% of the defects, and of these, glaucoma was the underlying cause in 81%; how applicable the findings are to a general neurologic practice is debatable. Most confrontation tests were relatively insensitive. All tests were more sensitive for posterior than anterior lesions. Although very commonly done, finger counting had a sensitivity of only 35%, but a specificity of 100%. The most sensitive single test was red comparison. Testing with a kinetic red target had the highest combined sensitivity and specificity of any individual test. The combination of kinetic testing with a red target combined with static finger wiggle was the best combination, with a sensitivity of 78% while retaining a specificity of 90%. The combination was significantly better than any single test. Description of the examiner’s face and finger counting, while simple tests, had low sensitivity and negative predictive values; it was recommended these tests not be used in isolation to exclude VF loss.

By convention, VFs are depicted as seen by the patient (i.e., right eye drawn on the right). This convention is backward from most things in clinical medicine, and violations of the rule occur sufficiently often that labeling notations are prudent. When confrontation fields are not adequate for the clinical circumstances, formal fields are done. These might include tangent screen examination, kinetic perimetry, or computerized automated static perimetry (Box 13.3).

BOX 13.3

Formal Visual Field Testing

Perimetry is the measurement of the visual field (VF) on a curved surface. Campimetry is the measurement of the VF on a flat surface. The tangent screen is the standard method for performing campimetry. For tangent screen examination, a black screen, blackboard, or other flat surface is used to examine the central 30 degrees of vision. The central fields can be evaluated more accurately with the tangent screen, the peripheral fields more accurately with perimetry. The patient is seated 1 to 2 m from the tangent screen; objects of various sizes and colors are brought into view using a black wand that blends into the background. Testing is now often done with a laser pointer. The test object is the only thing of visual interest against the black background. As with perimetry, the notation numerator is the test object size and the denominator the distance from the screen, often followed by a letter to indicate the target color. A field notation of 2/1,000 r indicates the field was done with a 2-mm red test object, and the patient was seated 1 m away from the screen. The tangent screen is especially valuable for measuring the size of the physiologic blind spot and for demonstrating central defects. Defects may be easier to detect when the VF is done at 2 m, because the dimensions of the field and the dimensions of the defect are doubled. The Amsler grid is another sensitive method for testing the central 10 degrees of the VF (Figure 13.17).

A perimeter is useful for testing the peripheral VFs, which cannot be done by tangent screen. Many different types of perimeters and perimetric techniques have been described. Perimetry may be kinetic or static. Kinetic perimetry entails moving a test object along various meridians and noting when it is detected. For standard kinetic perimetry (e.g., Goldmann), the patient gazes at a fixation point and various test objects are brought into the field of vision through multiple meridians in a hemispheric dome. White and colored test objects varying in size from 1 to 5 mm are used. The points at which a target of given size and color is first seen are recorded, and a line is drawn joining these points to outline the VF. The line representing the limits of the field for a given size and color test object is called an isopter. The smaller the test object, the smaller the VF. Mapping isopters for test objects of varying sizes and colors creates an image resembling a topographic map. Perimetric readings are expressed in fractions; the numerator indicates the target size and the denominator the distance away from the patient in millimeters. If the size of a VF defect is the same with all test objects, it is said to have steep, or abrupt, margins. If the defect is larger with smaller test objects, its margins are said to be gradual, or sloping, in character.

Limits of the VF vary according to the size, color, and brightness of the test object, the intensity of illumination, the state of adaptation of the eye, and the cooperation of the patient. The VF for a colored test object is smaller than the VF for a white object of the same size. The size of the VF is different for different colors. Changes in color fields precede gross field changes (color desaturation). Altered responses to color may help differentiate between retinal lesions and neurologic conditions. Formal fields provide permanent objective documentation of the VFs. They may be repeated periodically to look for progression or improvement.

The Goldmann perimeter uses a kinetic paradigm. Modern quantitative automated perimetry uses static perimeters, and automated perimetry has largely replaced manual perimetry. Static perimetry measures the threshold for perception of various targets at various locations in the VF with the aid of a computer and statistical analysis. The Humphrey Visual Field Analyzer is in widespread use in the United States. Statistical analysis of the VF data allows determination of the probability that a VF is normal. Automated perimetry is very sensitive for detecting VF defects. However, normal patients may appear to have an abnormal VF because of the large number of erroneous responses that can occur during automated testing. The instruments include reliability indices determined by the false-positive and false-negative responses (Figure 13.18).

FIGURE 13.18 Top.VF performed on a Goldmann perimeter in a patient with a chiasmal lesion. Bottom.Humphrey perimeter field in the same patient. (Reprinted from Beck RW, Bergstrom TJ, Lichter PR. A clinical comparison of visual field testing with a new automated perimeter, the Humphrey Field Analyzer, and the Goldmann perimeter. Ophthalmology 1985;92[1]:77–82. Copyright © 1985 American Academy of Ophthalmology, Inc. With permission.)

Visual Field Abnormalities

For neurologic purposes, VF abnormalities can be divided into scotomas, hemianopias, altitudinal defects, and concentric constriction or contraction of the fields. Figure 13.19 depicts some examples of different types of field defects. Because of the anatomy and organization of the visual system, neurologic disorders tend to produce straight-edged defects that respect either the horizontal or vertical meridian or have a characteristic shape because of the arrangement of the NFL. Respect of the horizontal meridian may occur because of the horizontal temporal raphe and the arching sweep of NFL axons above and below the macula. This pattern is characteristic of optic nerve, optic disk, and NFL lesions. The vascular supply of the retina consists of superior and inferior branches of the central retinal artery, which supply the upper and lower retina, respectively. Vascular disease characteristically causes altitudinal field defects that are sharply demarcated horizontally. The calcarine cortex is organized into a superior and an inferior bank, and lesions involving only one bank may produce VF defects that respect the horizontal meridian. The vertical meridian is respected because of the division into nasal and temporal hemiretinas that occurs at the chiasmal decussation and is maintained through the retrochiasmal visual pathways.

FIGURE 13.19 Types of VF defects. A. Central scotoma. B. Cecocentral scotoma. C. Junctional scotoma. D. Homonymous scotomas. E. Heteronymous scotomas. F. Right homonymous hemianopia. G. Bitemporal hemianopia. H. Congruous right homonymous hemianopia. I. Incongruous right homonymous hemianopia. J. Right superior quadrantanopia (“pie in the sky”). K. Right inferior quadrantanopia. L. Macular-sparing right homonymous hemianopia.

Scotomas

A scotoma (Gr. “darkness”) is an area of impaired vision in the field, with normal surrounding vision. With an absolute scotoma, there is no visual function within the scotoma to testing with all sizes and colors of objects. With a relative scotoma, visual function is depressed but not absent; smaller objects and colored objects are more likely to detect the abnormality. A positive scotoma causes blackness or a sense of blockage of vision, as though an object were interposed; it suggests disease of the retina, especially the macula or choroid. Positive scotomas are often due to exudate or hemorrhage involving the retina or opacity in the media. A negative scotoma is an absence of vision, a blank spot as if part of the field had been erased; it suggests optic nerve disease but can occur with lesions more posteriorly. With a negative scotoma, the defect may not be perceived until a VF examination is done.

A scotoma can often be demonstrated on confrontation VF testing using small objects and carefully exploring the central fields, but they are best demonstrated by the use of the tangent screen. The physiologic blind spot (Mariotte’s spot) is a scotoma corresponding to the optic nerve head, which contains no rods or cones and is blind to all visual impressions. The physiologic blind spot is situated 15 degrees lateral to and just below the center of fixation because the disk lies nasal to the macula and the blind spot is projected into the temporal field. Elliptical in shape, it averages 7 to 7½ degrees vertically and 5 to 5½ degrees horizontally and extends 2 degrees above and 5 degrees below the horizontal meridian. On a tangent screen with the patient 1 m away using a 1-mm white object, the average measurements for the blind spot are from 9 to 12 cm horizontally and 15 to 18 cm vertically. The blind spot is enlarged in papilledema and ON.

Scotomas are described by their location or their shape. A central scotoma involves the fixation point and is seen in macular or optic nerve disease. It is typical for ON but can occur in vascular and compressive lesions (Figure 13.19A). A paracentral scotoma involves the areas adjacent to the fixation point, and it has the same implications as for a central scotoma. A cecocentral scotoma extends from the blind spot to fixation. It is usually accompanied by loss of all central vision with preservation of a small amount of peripheral vision, and it strongly suggests optic nerve disease (Figures 13.19B and 13.20). Central, paracentral, and cecocentral scotomas are all suggestive of a process involving the PMB. Any scotoma involving the blind spot implies optic neuropathy.

FIGURE 13.20 Bilateral cecocentral scotomas in a patient with bilateral optic neuritis.

An arcuate scotoma is a crescent defect arching out of the blind spot, usually because of optic neuropathy with the brunt of damage falling on the fibers forming the superior and inferior NFL arcades. A nasal step defect is a scotoma that involves the nasal part of the VF away from fixation, usually respecting the horizontal meridian, which is due to optic neuropathy and often progresses to become a broad arcuate scotoma. Nasal step defects are common, especially in optic neuropathy because of glaucoma. A junctional scotoma is an optic nerve defect in one eye (central, paracentral, or cecocentral scotoma) and a superior temporal defect in the opposite eye (syndrome of Traquair). This is due to a lesion (usually a mass) that involves one optic nerve close to the chiasm, which damages the inferior nasal fibers from the opposite eye (Wilbrand’s knee) as they loop forward into the proximal optic nerve on the side of the lesion (Figures 13.9 and 13.19C). The temporal VF defect in the contralateral eye may be subtle and easily missed. The anatomic evidence supporting the existence of Wilbrand’s knee has been questioned, but clinical cases continue to suggest it exists. No junctional scotoma could be detected in three patients whose optic nerves were surgically divided at the optic nerve-chiasm junction.

Although scotomas most often result from disease of the retina or optic nerve, they may also be caused by cerebral lesions. Occipital pole lesions primarily affecting the macular area can produce contralateral homonymous hemianopic scotomas (Figure 13.19D). Because the bulk of fibers in the chiasm come from the macula, early compression may preferentially affect central vision producing bitemporal heteronymous paracentral scotomas (Figure 13.19E); with progression of the lesion, a full blown bitemporal hemianopia will appear (Figure 13.19G). Optic nerve lesions such as glioma and drusen (hyaline excrescences that may be buried in or on the surface of the nerve) may cause scotomas, contraction of the VFs, or sector defects. Enlargement of the physiologic blind spot is referred to as a peripapillary scotoma.

Other types of scotomas occur from primary ocular disease, such as retinitis, chorioretinitis, and glaucoma, which are not related directly to disease of the nervous system (Box 13.4).

BOX 13.4

Other Types of Scotomas

Glaucoma may cause arcuate, cuneate, comma-shaped, or other partially ring-shaped scotomas. A Seidel scotoma arises from the blind spot and has a thin and well-demarcated arcuate tail, the shape resembling a comma. A Bjerrum scotoma is shaped like a bow, extending from the blind spot to near fixation. Both are common in glaucoma. Peripheral scotomas may be present anywhere in the field of vision. In annular, or ring, scotomas, there is a loss of vision in a doughnut shape with relative sparing of fixation and of the far periphery. These types of scotomas are typically due to retinitis pigmentosa, a condition primarily affecting the rods that are concentrated in the midzone of the retina. Ring scotomas also occur in optic neuropathy, macular lesions, cancer-associated retinopathy, choroiditis, and myopia. Many ophthalmologic conditions produce a global depression of retinal function and concentric constriction of the VF, rather than a discrete defect. Ophthalmoscopic examination generally reveals the nature of such conditions.

Subjective scotomas cannot be delineated in the field examination. Subjective scotomas include the scintillating scotomas, or teichopsias, of migraine, and the annoying but harmless vitreous floaters that many normal individuals experience.

Hemianopia

Hemianopia is impaired vision in half the VF of each eye; hemianopic defects do not cross the vertical meridian. Hemianopias may be homonymous or heteronymous. A homonymous hemianopia causes impaired vision in corresponding halves of each eye (e.g., a right homonymous hemianopia is a defect in the right half of each eye). Homonymous hemianopias are caused by lesions posterior to the optic chiasm, with interruption of the fibers from the temporal half of the ipsilateral retina and the nasal half of the contralateral retina. Vision is lost in the ipsilateral nasal field and the contralateral temporal field (Figure 13.21). A heteronymous hemianopia is impaired vision in opposite halves of each eye (e.g., the right half in one eye and the left half in the other). Unilateral homonymous hemianopias, even those with macular splitting, do not affect visual acuity. Patients can read normally with the preserved half of the macula, but those with left-sided hemianopias may have trouble finding the line to be read. Occasionally, patients with homonymous hemianopia will read only half of the line on the acuity chart.

FIGURE 13.21 Macular-splitting right homonymous hemianopia in a patient with a neoplasm of the left occipital lobe.

A homonymous hemianopia may be complete or incomplete. If incomplete, it may be congruous or incongruous. A congruous hemianopia shows similar-shaped defects in each eye (Figure 13.19H). The closer the optic radiations get to the occipital lobe, the closer lie corresponding visual fibers from the two eyes. The more congruous the field defect, the more posterior the lesion is likely to be. An incongruous hemianopia is differently shaped defects in the two eyes (Figure 13.19I). The more incongruous the defect, the more anterior the lesion. The most incongruous hemianopias occur with optic tract and lateral geniculate lesions. With a complete hemianopia, congruity cannot be assessed; the only localization possible is to identify the lesion as contralateral and retrochiasmal. A superior quadrantanopia implies a lesion in the temporal lobe affecting Meyer’s loop (inferior retinal fibers): “pie in the sky” (Figure 13.19J). Such a defect may occur after temporal lobe epilepsy surgery because of damage to the anteriorly looping fibers. An inferior quadrantanopia (“pie on the floor”) implies a parietal lobe lesion affecting superior retinal fibers (Figure 13.19K). A macular-sparing hemianopia is one that spares the area immediately around fixation; it implies an occipital lobe lesion (Figure 13.19L). The explanation for macular sparing remains unclear. There has been conjecture about dual representation of the macula in each occipital pole, but this has never been confirmed anatomically. More likely it is collateral blood supply from the anterior or middle cerebral artery, which protects the macular region from ischemia. Or it could simply be that the extensive cortical representation of the macula both at the occipital pole and anteriorly in the depths of the calcarine fissure makes it difficult for a single lesion to affect all macular function. A small amount of macular sparing may be due to fixation shifts during testing.

Incomplete homonymous VF defects are common. These include partial or irregular defects in one or both of the hemifields, relative rather than absolute loss of vision, an inability to localize the visual stimulus, and hemianopia only for objects of a certain color (hemiachromatopsia). Extinction (visual inattention) is hemianopic suppression of the visual stimulus in the involved hemifield when bilateral simultaneous stimuli are delivered. Visual extinction is most characteristic of lesions involving the nondominant parietooccipital region. Riddoch’s phenomenon is a dissociation between the perception of static and kinetic stimuli. The patient may not perceive a stationary object but detect it instantly when it moves.

Heteronymous hemianopias are usually bitemporal; only rarely are they binasal. A bitemporal hemianopia is usually due to chiasmatic disease, such as a pituitary tumor growing up out of the sella turcica and pressing on the underside of the chiasm (Figure 13.18). Bitemporal field defects can usually be detected earliest by demonstrating bitemporal desaturation to red. Because of the anterior inferior position of decussating inferior nasal fibers, lesions impinging from below produce upper temporal field defects, which evolve into a bitemporal hemianopia (Figure 13.6). Lesions encroaching from above tend to cause inferior temporal defects initially. The defect will be first and worst in the upper quadrants with infrachiasmatic masses (e.g., pituitary adenoma), and it will be first and worst in the lower quadrants with suprachiasmatic masses (e.g., craniopharyngioma). Patients with postfixed chiasms and pituitary tumors may present with optic nerve defects, and those with prefixed chiasms may have optic tract defects.

The most common cause of bitemporal hemianopia is a pituitary adenoma; occasionally, it results from other parasellar or suprasellar lesions such as meningioma and craniopharyngioma, as well as glioma of the optic chiasm, aneurysms, trauma, and hydrocephalus. Other VF defects that may simulate bitemporal hemianopia include tilted optic disks, bilateral cecocentral scotomas, and bilaterally enlarged blind spots. Binasal hemianopias may occur from disease impinging on the lateral aspect of the chiasm bilaterally (e.g., bilateral intracavernous carotid aneurysms), but they are more likely to be due to bilateral optic neuropathy.

An altitudinal VF defect is one involving the upper or lower half of vision, usually in one eye, and usually due to retinal vascular disease (central retinal artery or branch occlusion or anterior ischemic optic neuropathy [AION]). A partial altitudinal defect may approximate a quadrantanopia. Altitudinal defects do not cross the horizontal meridian.

Constriction of the VFs is characterized by a narrowing of the range of vision, which may affect one or all parts of the periphery. Constriction may be regular or irregular, concentric or eccentric, temporal or nasal, and upper or lower. Symmetric concentric contraction is most frequent and is characterized by a more or less even, progressive reduction in field diameter through all meridians. Such constriction is referred to as funnel vision, as opposed to tunnel vision (see below). Concentric constriction of the VFs may occur with optic atrophy, especially secondary to papilledema or late glaucoma, or with retinal disease, especially retinitis pigmentosa. Narrowing of the fields because of fatigue, poor attention, or inadequate illumination must be excluded, as must spurious contraction because of decreased visual acuity or delayed reaction time. Slight constriction of the VF may occur when there is a significant refractive error. Diffuse depression is the static perimeter equivalent of constriction on kinetic perimetry.

Concentric constriction of the fields is sometimes seen in hysteria. A suspicious finding is when the fields fail to enlarge as expected with testing at increasing distance (tubular or tunnel fields). Normally, the field of vision widens progressively as the test objects are held farther away from the eye. However, in nonorganicity, this normal widening is not seen, and the entire width of the field is as great at 1 ft from the eye as it is at 2, or 15 ft. The normal VF is a funnel; the nonorganic VF is a tunnel. The tubular field can be demonstrated either by testing the extent of the VF at varying distances from the patient, or it can be shown by using test objects of different sizes at a constant distance. Spiral contraction is a progressive narrowing of the VF during the process of testing. It may be a sign of nonorganicity, but it is probably more suggestive of fatigue. A similar pattern field is the star-shaped VF, where there is an irregularity of outline. This may be seen in nonorganicity, fatigue, or poor attention.

The Ophthalmoscopic Examination

The physician using a direct ophthalmoscope is like a one-eyed Eskimo peering into a dark igloo from the entryway with a flashlight. Only a narrow sector of the posterior pole is visible, and there is no stereopsis. Pupil dilation significantly increases the field of view. Indirect ophthalmoscopy, used by ophthalmologists, can stereoscopically view almost the entire vista of the fundus. PanOptic direct ophthalmoscopes (Welch-Allyn) give the advantage of a broader view but still reveal only the posterior pole. See Box 13.5 for a brief discussion of the techniques of direct ophthalmoscopy. It is important to become facile by practicing direct ophthalmoscopy on all patients, as the examination is inevitably most technically difficult in situations where fundus examination is most critical.

BOX 13.5

Direct Ophthalmoscopy

The standard direct ophthalmoscope has dials that adjust the light apertures and filters and allow the examiner to focus. The small aperture is for examining an undilated pupil, the large aperture for examining a dilated pupil. Using the small aperture may help minimize reflections from the cornea. The red-free filter is useful for examining blood vessels, looking for hemorrhages, and examining the nerve fiber layer (NFL). The red reflex can be assessed from a distance of 12 to 15 in. Opacities in the media (e.g., cataract) appear as black dots against the red background. The ocular fundus is the only place in the body where blood vessels can be visualized directly. Changes in the retinal vasculature in conditions such as diabetes and hypertension mirror the status of the systemic circulation. The fundus may also reveal important findings in systemic diseases such as endocarditis and AIDS.

In the neurologic examination, the areas of primary concern are the disk, the macula, and the arteries. The disk is normally round or a vertically oriented slight oval. The nasal margin is normally slightly blurred compared to the temporal. The disk consists of a peripheral neuroretinal rim and a central cup. The neuroretinal rim consists of axons streaming from the retina to enter the optic nerve. The physiologic cup is a slight depression in the center of the disk that is less pinkish than the rim and shows a faint latticework because of the underlying lamina cribrosa. The rim is elevated slightly above the cup. The cup normally occupies about a third of the temporal aspect of the disk. To locate the disk, a helpful technique is to find a retinal blood vessel, focus on it, and then follow it to the disk. In severe myopia, the disk may appear larger and paler than normal. In the aphakic eye, the disk looks small and far away.

The myelinated axons making up its substance render the normal optic disk yellowish-white. It is paler temporally where the papillomacular bundle (PMB) enters. The normal disk lies flat and well demarcated against the surrounding retina, with arteries and veins crossing the margins and capillaries staining the surface a faint pink. The size of the scleral opening varies from individual to individual. When the opening is small, the disk consists entirely of neuroretinal tissue, and the cup is inconspicuous or nonexistent. Such a small cupless disk is more vulnerable to anterior ischemic optic neuropathy and is termed a disk at risk. The normal cup-to-disk ratio is about 0.1 to 0.5. In patients with glaucoma, the cup-to-disk ratio is increased and the cup is more prominent and often nasally displaced.

The central retinal artery enters the eye through the physiologic cup and divides into superior and inferior branches, which in turn divide into nasal and temporal branches, yielding four prominent arterial trunks emanating from the disk. Beyond the second branch, the retinal vessels are arterioles, visible because of the 14× magnification provided by the patient’s lens and cornea. Cilioretinal arteries are present in many normal individuals. These vessels arise from posterior ciliary arteries, enter the eye along the disk margin, and perfuse the peripapillary retina. They may become prominent as shunt vessels when there is optic nerve compression. Varying amounts of pigmentation are present in the retina near the temporal border of the disk, especially in dark-skinned persons. At times a pigment ring may completely surround the disk. White scleral and dark choroidal rings may sometimes be seen.

The macula is a dark area that lies about two disk diameters temporal to and slightly below the disk. The macula appears darker than the surrounding retina because the depression of the macula and fovea means the retina is thinner in that area, allowing more of the deeply colored choroid to show through. The area of the macula is devoid of large blood vessels. The fovea centralis appears as a pinpoint of light reflected from the center of the macula. The macula may be seen more easily with a red-free filter. It is sometimes easier to visualize the macula if the patient looks directly into the light.

The routine fundus examination in neurologic patients is generally done through the undilated pupil. The fundus examination is more challenging when the patient has a small pupil, myopia, or opacities in the media such as cataract. One or more of these are commonly present in older individuals. In some circumstances, the benefits of an adequate fundus exam outweigh the minimal risk of precipitating an attack of acute narrow angle glaucoma by using mydriatic drops. A crude estimate of the narrowness of the iridocorneal angle can be made by shining a light from the temporal side to see if a shadow is cast on the nasal side of the iris and sclera. The risk of an attack of acute narrow-angle glaucoma because of the use of mydriatic drops has been estimated at 0.1%. Mydriatic drops are best avoided in situations where assessment of pupillary function is critical, such as patients with head injury or other causes of depressed consciousness. Their use in such situations must be obtrusively documented, even to the point of writing “eye drops in” on the patient’s forehead.

Localization and Disorders of Visual Function

Disorders of the afferent visual system can be divided into prechiasmal, chiasmal, and retrochiasmal. Disease in each of these locations has characteristic features that usually permit its localization. The etiologic processes affecting these different segments of the afferent visual system are quite different. As a generalization, prechiasmal lesions cause monocular visual loss; impaired color perception; a central, paracentral, or cecocentral VF defect; and an APD. The disk may or may not appear abnormal depending on the exact location of the lesion. Chiasmal lesions cause heteronymous VF defects, most often bitemporal hemianopia, with preservation of visual acuity and color perception and a normal appearing optic disk. Retrochiasmal lesions cause a contralateral homonymous hemianopia and have no effect on acuity or disk appearance. There is usually no effect on color vision, but some central lesions may cause achromatopsia. A summary of the features of disease involving the macula, optic nerve, chiasm, optic tract, LGB, optic radiations, and calcarine cortex can be found in Table 13.1.

TABLE 13.1 Clinical Characteristics of Acute Lesions Involving Different Parts of the Afferent Visual Pathway

AION, anterior ischemic optic neuropathy; APD, afferent, papillary defect; decr, decreased; LHON, Leber hereditary optic neuropathy; MS, multiple sclerosis; OKN, optokinetic nystagmus; ON, optic neuritis.

Prechiasmal Lesions

Prechiasmal disorders affect the optic nerve. Disorders can be divided into those that affect the disk (papillopathy) and those that affect the retrobulbar segment between the globe and the chiasm. The macula gives rise to the majority of the fibers in the optic nerve, and disease of the macula itself can cause a clinical picture that is at times difficult to distinguish from optic neuropathy. Common causes of maculopathy include age-related macular degeneration and central serous retinopathy (Table 13.1). Macular disease causes marked impairment of central acuity and impaired color vision. There may be a central scotoma. A distinct central scotoma with normal field between the central defect and the blind spot is more common in macular than in optic nerve disease. Macular disease often causes metamorphopsia, a distortion of visual images. When severe, maculopathy can cause an APD. Prolongation of the time to recover vision after direct, intense light stimulation (photostress test) can sometimes help to distinguish macular from optic nerve disease (Box 13.6). Other retinal lesions severe enough to cause monocular VF defects are almost all visible ophthalmoscopically.

BOX 13.6

The Photostress Test

In macular disease, the photoreceptors require longer to recover from bleaching of the retinal pigments after exposure to a bright light. The photostress test is done by determining a baseline visual acuity, then shining a bright light (e.g., a fresh penlight) into the eye for 10 seconds, and then determining the time required for the visual acuity to return to baseline. Reliable reference values are not available; the test is mainly useful with unilateral disease when the unaffected eye can be used for comparison. In optic nerve disease, the photostress test is normal. Recovery times may reach several minutes in macular disorders such as macular edema, central serous retinopathy, and macular degeneration.

A macular star is a radial pattern of exudates in the perimacular retina. They are common in hypertension, papilledema, and in other conditions. Neuroretinitis refers to the association of ON with a macular star and is commonly of viral origin. Chorioretinitis is inflammation involving choroid and retina, which is most often due to infections such as tuberculosis, syphilis, toxoplasmosis, cytomegalovirus, and HIV. Chorioretinitis often leaves whitish scars surrounded by clumps of pigment. Cytomegalovirus chorioretinitis is common in AIDS.

Monocular altitudinal defects are characteristic of disease in the distribution of the central retinal artery. Central vision may be spared because the macula is often perfused by the cilioretinal arteries. AION (see below) is another cause of an altitudinal defect. Bilateral altitudinal defects may occur with bilateral lesions in certain parts of the visual pathway, for example, bilateral occipital infarction or a large prechiasmal lesion compressing both optic nerves. A checkerboard pattern is a superior altitudinal defect in one eye and an inferior altitudinal defect in the other eye.

Disorders of the Optic Disk

The color and appearance of the disk may change in a variety of circumstances. The disk may change color—to abnormally pale in optic atrophy or to abnormally red with disk edema. The margins may become obscured because of disk edema or the presence of anomalies. Edema of the disk is nonspecific. It may reflect increased intracranial pressure, or it may occur because of optic nerve inflammation, ischemia, or other local processes. By convention, disk swelling because of increased intracranial pressure is referred to as papilledema; under all other circumstances, the noncommittal terms disk edema or disk swelling are preferred. Visual function provides a critical clue to the nature of disk abnormalities. Patients with acute papilledema and those with disk anomalies have normal visual acuity, VFs, and color perception. Impairment of these functions is the rule in patients suffering from optic neuropathies of any etiology. The first step in evaluating a questionably abnormal disk is therefore a careful assessment of vision.

Papilledema

Increased intracranial pressure exerts pressure on the optic nerves, which impairs axoplasmic flow and produces axonal edema and an increased volume of axoplasm at the disk. The swollen axons impair venous return from the retina, engorging first the capillaries on the disk surface, then the retinal veins, and ultimately causing splinter- and flame-shaped hemorrhages as well as cotton wool exudates in the retinal NFL. Further axonal swelling eventually leads to elevation of the disk above the retinal surface. Transient visual obscurations, momentary graying out or blacking out of vision, often precipitated by postural changes, are classical symptoms of papilledema, especially in pseudotumor cerebri (idiopathic intracranial hypertension [IIH]). Obscurations may be due to microvascular compromise at the nerve head.

The four stages of papilledema are early, fully developed, chronic, and atrophic. Fully developed papilledema is obvious, with elevation of the disk surface, humping of vessels crossing the disk margin, obliteration of disk margins, peripapillary hemorrhages, cotton wool exudates, engorged and tortuous retinal veins, and marked disk hyperemia. The recognition of early papilledema is much more problematic (Figure 13.22). Occasionally, the only way to resolve the question of early papilledema is by serial observation. The earliest change is loss of previously observed spontaneous venous pulsations (SVPs). Venous pulsations are best seen where the large veins dive into the disk centrally. The movement is a back-and-forth rhythmic oscillation of the tip of the blood column, which resembles a slowly darting snake’s tongue. Side-to-side expansion of a vein is much more difficult to see. The presence of SVPs indicates an intracranial pressure less than approximately 200 mm H₂O. However, because they are absent in 10% to 20% of normals, only the disappearance of previously observed SVPs is clearly pathologic.

FIGURE 13.22 Early papilledema.

As papilledema develops, increased venous back pressure dilates the capillaries on the disk surface, transforming its normal yellowish-pink color to fiery red. Blurring of the superior and inferior margins evolves soon after. However, because these margins are normally the least distinct areas of the disk, blurry margins alone are not enough to diagnose papilledema. There is no alteration of the physiologic cup with early papilledema. With further evolution, the patient with early papilledema will develop diffuse disk edema, cup obscuration, hemorrhages, exudates, and venous engorgement. Frank disk elevation then ensues as the fundus ripens into fully developed papilledema (Figure 13.23). In chronic papilledema, hemorrhages and exudates resolve and leave a markedly swollen “champagne cork” disk bulging up from the plane of the retina. If unrelieved, impaired axoplasmic flow eventually leads to death of axons and visual impairment, which evolves into the stage of atrophic papilledema, or secondary optic atrophy. Papilledema ordinarily develops over days to weeks. With acutely increased intracranial pressure because of subarachnoid or intracranial hemorrhage, it may develop within hours. Measuring diopters of disk elevation ophthalmoscopically has little utility.

FIGURE 13.23 Severe papilledema.

The changes in the optic nerve head in papilledema are both mechanical and vascular in nature. The mechanical signs of disk edema include blurring of the disk margins, filling in of the physiologic cup, protrusion of the nerve head, edema of the NFL and retinal or choroidal folds, or both. The vascular signs include venous congestion, hyperemia of the nerve head, papillary and peripapillary hemorrhages, NFL infarcts (cotton-wool spots), and hard exudates of the optic disk.

Acute papilledema causes no impairment of visual acuity or color vision. The typical patient has no symptoms related to its presence except for obscurations. The blind spot may be enlarged, but VF testing is otherwise normal. In patients who develop optic atrophy following papilledema, the visual morbidity can be severe and may include blindness.

With current technology, imaging has usually detected intracranial mass lesions before the development of increased intracranial pressure. As a result, IIH is the most common cause of papilledema in the developed world. IIH can occur without papilledema, or with asymmetric papilledema, rarely with unilateral papilledema. The typical patient with IIH is an obese, young female with headaches, no focal findings on neurologic examination, normal imaging except for small ventricles, and normal CSF except for elevated opening pressure. Without adequate treatment, visual loss is a common sequel.

Other Causes of Disk Edema

Changes ophthalmoscopically indistinguishable from papilledema occur when conditions primarily affecting the optic nerve papilla cause disk edema. Papilledema is usually bilateral; other causes of disk edema are often unilateral (Table 13.2). Optic neuropathies generally cause marked visual impairment, including loss of acuity, central or cecocentral scotoma, loss of color perception, and an APD. Disease of the optic nerve head is usually due to demyelination, ischemia, inflammation, or compression. ON and AION are two common conditions that cause impaired vision and disk edema. Both are usually unilateral. Compressive lesions of the optic nerve in the orbit may cause disk edema, but intracanalicular and intracranial compression usually does not. ON with disk edema is sometimes called papillitis. Papillitis may occur as an isolated abnormality, as a manifestation of multiple sclerosis (MS), or as a complication of some systemic illness. Demyelinating optic neuropathies causing papillopathy are common as a feature of MS, but they also can occur as an independent disease process or complicate other disorders such as acute disseminated encephalomyelitis and neuromyelitis optica (NMO), which includes Devic’s disease. There are many other causes of optic neuropathy; some of the more common conditions are listed in Table 13.3.

TABLE 13.2 Some Causes of Unilateral Disk Edema

TABLE 13.3 Some Causes of Optic Neuropathy

Optic Neuritis

Inflammation or demyelination of the optic nerve can occur in a variety of conditions, including MS, postviral syndromes, sarcoidosis, collagen vascular disease, neurosyphilis, and others. Many cases are idiopathic. The majority of patients are women in the 20 to 50 age range. ON occurs sometime during the course of MS in 70% of patients and is the presenting feature in 25%. Some 50% to 70% of patients presenting with ON eventually develop other evidence of MS. Factors that increase the likelihood of underlying MS in patients with ON include the presence of Uhthoff’s phenomenon (increased symptoms with elevation of body temperature or after exercise), HLA-DR2 positivity, and a recurrent episode. Decreased acuity, impaired color perception, central or cecocentral scotoma, disk edema, and an APD are the typical findings. For a video of an APD, see Video Link 13.1 (from Kathleen B. Digre, MD at the University of Utah Neuro-Ophthalmology Virtual Education Library: NOVEL). Color vision loss usually parallels acuity loss, but in ON, the loss of color vision may be more severe than expected for the loss of acuity. Visual loss in ON occurs suddenly and tends to progress over 1 to 2 weeks, with substantial recovery over 2 to 12 weeks. Severe visual loss acutely does not necessarily portend poor recovery. Eye pain is present in 90% of patients, and many have positive visual phenomena with colors or flashing lights (photopsias, phosphenes). The eye pain is usually mild but can become severe and more debilitating than the visual loss. The pain may precede or begin concomitantly with the visual loss and is usually worsened with eye movement, particularly upgaze. The absence of pain suggests a noninflammatory type of optic neuropathy. In about 65% of cases of ON, the disk appears normal; in the remainder, there is mild disk edema and occasional NFL hemorrhages. Pain can occur whether or not there is disk edema. Optic atrophy ensues over the next several weeks in 50% of patients. Improvement to normal or near normal acuity occurs in 90% of patients. ON may rarely involve the chiasm (chiasmal neuritis).

In NMO, there are lesions of the optic nerves and the spinal cord. It is a distinct entity from MS, but separating the two clinically may be difficult. The spinal cord lesion extends over three or more vertebral segments (longitudinally extensive transverse myelitis [LETM]). The spinal cord syndrome is usually sudden and severe and may be permanent. In one series of 60 patients, ON was the initial feature in 53.3%.

NMO is increasingly being seen as a spectrum of neurologic conditions defined by serologic tests. The term NMO spectrum disorder (NMOSD) emphasizes that patients with ON or LETM alone are often antibody positive and that some patients have brain lesions. The brain MRI abnormalities differ from those typical of MS. Autoantibodies against aquaporin-4 and myelin-oligodendrocyte glycoprotein have been identified so far.

Anterior Ischemic Optic Neuropathy

AION is the most common syndrome of optic nerve ischemia, and the most common optic neuropathy in adults over 50 after glaucoma. In AION, microangiopathy produces occlusion of the short posterior ciliary arteries and infarction of all or part of the disk. Visual loss is sudden, painless, nonprogressive, and generally does not improve. Decreased acuity, impaired color perception, an altitudinal field defect, usually inferior, and pallid disk edema are the typical findings acutely; evolving subsequently into optic atrophy. In the acute phase, a pale disk with hemorrhage will virtually always be due to AION. Other useful findings suggesting AION are altitudinal swelling and arterial attenuation.

AION is due to disease involving the posterior ciliary arteries, not the central retinal artery, and is divided into two forms: arteritic and nonarteritic. Arteritic AION most commonly complicates giant cell arteritis (GCA), accounting for about 10% to 15% of patients. Usually, these patients are over 65 and have more severe visual loss than patients with nonarteritic AION. A history of headache, jaw claudication, and scalp tenderness is very suspicious. Evidence of polymyalgia rheumatica, such as malaise, weight loss, myalgias, and an elevated ESR, increases the likelihood that AION is due to GCA. In a meta-analysis of 21 studies, jaw claudication and diplopia were the only historical features that substantially increased the likelihood of GCA. Predictive physical findings included temporal artery beading, prominence, and tenderness; the absence of any temporal artery abnormality was the only clinical factor that modestly reduced the likelihood of disease. There is evidence that varicella-zoster virus infection may trigger the inflammatory cascade that characterizes GCA.

Premonitory amaurosis fugax is more common in the arteritic form. They do not have a small disk in the fellow eye (disk at risk, see below). Involvement of the opposite eye occurs in approximately 15% of patients within 5 years. Although no treatment affects the outcome in the involved eye, recognition and management of underlying vasculitis may prevent a future attack in the opposite eye.

Nonarteritic AION is most often caused by a microvasculopathy related to hypertension, diabetes, tobacco use, arteriosclerosis, or atherosclerosis. Some cases are due to impaired microvascular perfusion related to systemic hypotension or increased intraocular pressure. There is a syndrome of posterior ischemic optic neuropathy, lacking disk edema, but it is rare and much less well defined than the anterior ischemic syndromes. It may be difficult to distinguish ON from ischemic optic neuropathy. In contrast to ON, the visual loss in AION is usually permanent, although one-third of patients may improve somewhat.

Other Optic Neuropathies

Numerous other conditions may affect the optic nerve head, causing visual loss and disk abnormalities (e.g., glaucoma; LHON and other hereditary optic atrophies; toxins and drugs; primary and metastatic tumors; malnutrition and deficiency states; neurodegenerative disorders; leukodystrophies; sarcoid; optic perineuritis; and congenital anomalies). Dysthyroid optic neuropathy occurs as a late complication of thyroid orbitopathy when enlarged ocular muscles compress the nerve at the orbital apex. See Table 13.3. It is important to distinguish ON from compressive lesions of the optic nerve. One characteristic feature of compressive optic neuropathy is that the condition continues to progress, often insidiously. Large, abnormal-appearing veins on the disk surface because of collateral venous drainage between the retinal and ciliary venous systems (optociliary shunt vessels) may provide a telltale clue to a compressive lesion (Figure 13.24). The triad of progressive visual loss, optic atrophy, and optociliary shunt vessels is highly suggestive.

FIGURE 13.24 Pale, elevated optic disk with optociliary shunt vessels in a blind eye; the typical findings of an optic nerve meningioma. (Reprinted from Savino PJ, Danesh-Meyer HV; Wills Eye Hospital [Philadelphia, PA]. Neuro-Ophthalmology. 2nd ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2012, with permission.)

Pseudopapilledema

Some conditions affecting the nerve head cause striking disk changes of little or no clinical import. This circumstance arises frequently when routine ophthalmoscopy unexpectedly reveals an abnormal-appearing disk in a patient with migraine or some seemingly benign neurologic complaint. Such patients generally have normal vision and no visual complaints. Common causes of pseudopapilledema include optic nerve drusen and myelinated nerve fibers.

Optic nerve drusen, or hyaloid bodies, are acellular, calcified hyaline deposits within the optic nerve that may elevate and distort the disk (Figure 13.25). Drusen occur in about 2% of the population and are bilateral in 70% of cases. They are familial, inherited as an irregular dominant with incomplete penetrance, and occur almost exclusively in Caucasians. On the disk surface, drusen have a highly refractile, rock-candy appearance. But when buried beneath the surface, drusen may produce only disk elevation and blurred margins, causing confusion with papilledema. Optic nerve drusen are not to be confused with retinal drusen, which are an age-related abnormality consisting of yellowish-white, round spots of variable size concentrated at the posterior pole. Myelinated nerve fibers occasionally extend beyond the disk margin into the retina, which causes a very striking disk picture but signifies nothing (Figure 13.26). Other causes of pseudopapilledema include remnants of the primitive hyaloid artery (Bergmeister’s papilla), tilted disks (Figure 13.27), and extreme hyperopia.

FIGURE 13.25 Drusen of the optic nerve head simulating papilledema.

FIGURE 13.26 Myelinated nerve fibers.

FIGURE 13.27 The congenital tilted optic disk is apparent as the oval of nerve tissue superiorly. There is no apparent optic cup. (Reprinted from Chern KC, Saidel MA. Ophthalmology Review Manual. 2nd ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2012, with permission.)

Distinguishing pseudopapilledema from acquired disk edema can be difficult. Features that may be helpful include the following: in papilledema, the disk is usually hyperemic; the disk margin blurriness is at the superior and inferior poles early in the process; blood vessels look normal except for fullness of the veins; SVPs are absent; and the NFL is dull with the retinal blood vessels obscured because of retinal edema. In pseudopapilledema, the disk color remains normal; blurriness of the disk margin may be irregular, and the disk may have a lumpy appearance; SVPs are usually present; the blood vessels on the disk frequently look anomalous; and the NFL is clear. Hemorrhages are common in papilledema and extremely rare in pseudopapilledema. If in doubt, consult an ophthalmologist.

Optic Atrophy

In optic atrophy, the disk is paler than normal and more sharply demarcated from the surrounding retina, sometimes having a punched-out appearance (Figure 13.28). The disk margins stand out distinctly; the physiologic cup may be abnormally prominent and extend to the margin of the disk. Loss of myelinated axons and their supporting capillaries with replacement by gliotic scar produce the lack of color, which may vary from a dirty gray to a blue-white color to stark white. Loss of axons causes involution of the capillary bed of the disk and allows the sclera to show through, contributing to the pallor. Dark choroidal pigment deposits may be present about the margin of the disk. The depth of color of the choroid will influence the perception of the degree of contrast between the disk and retina. An atrophic disk may appear perceptibly smaller. Pallor of the temporal portion of the disk—a classical finding in MS—may precede definite atrophy, but normal physiologic temporal pallor makes this finding often equivocal.

FIGURE 13.28 Primary optic atrophy.

Optic atrophy may follow some other condition (ON, AION, or papilledema) and is then referred to as secondary or consecutive optic atrophy. Primary optic atrophy, appearing de novo, occurs as a heredofamilial condition (e.g., LHON) or after toxic, metabolic, nutritional, compressive, or glaucomatous insult to the nerve. Some causes of optic atrophy are listed in Table 13.4. The term cavernous, or pseudoglaucomatous, optic atrophy is used if there is marked recession of the disk. Glaucoma is a common cause of optic atrophy; it produces both an increase in the depth of the physiologic cup and atrophy of the nerve (Figure 13.29). LHON is an uncommon mitochondriopathy that affects only males; it may cause the appearance of disk edema acutely but evolves into optic atrophy. It typically affects young men and causes sudden unilateral visual loss with involvement of the fellow eye within days to months. Characteristic peripapillary telangiectasias are frequently present, even in the uninvolved eye. Bow-tie or band optic atrophy refers to pallor of the disk that may develop in an eye with temporal VF loss following a lesion of the optic chiasm or tract (Box 13.7, Figure 13.30).

TABLE 13.4 Some Causes of Optic Atrophy

FIGURE 13.29 Glaucomatous optic atrophy. (Courtesy Richard A. Lewis.)

BOX 13.7

Bow-Tie (Band) Optic Atrophy

The macula lies temporal to the disk, and fibers from the nasal hemimacula enter the temporal aspect of the disk. These papillomacular fibers are responsible for the normal pallor of the temporal aspect of the disk, and the pallor is accentuated with NFL axon loss. There is also atrophy of the nasal hemiretinal NFL. Fibers from the peripheral nasal hemiretina enter the nasal aspect of the disk, and axon loss causes nasal disk pallor. With axon loss involving both nasal hemimacula and nasal hemiretina, the result is a transverse band of atrophy across the disk. The appearance is reminiscent of a white bow tie.

FIGURE 13.30 “Band” or “bow-tie” atrophy of the right optic disk in a patient with a temporal hemianopia caused by a pituitary adenoma. Note horizontal band of atrophy across the right disk, with preservation of the superior and inferior portions of the disk. (Reprinted from Miller NR, Biousse V, Newman NJ, et al. Walsh and Hoyt’s Clinical Neuro-Ophthalmology: The Essentials. 2nd ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2008, with permission.)

A patient may have disk edema in one eye and optic atrophy in the other eye. Foster Kennedy syndrome is due to an olfactory groove meningioma, causing anosmia (see Chapter 12), with optic atrophy because of direct compression ipsilateral to the neoplasm, and late contralateral papilledema because of increased intracranial pressure. Optic atrophy in one eye with disk edema in the other eye is now much more commonly seen with AION or ON (pseudo–Foster Kennedy syndrome), when the disease strikes the opposite eye weeks to months after an initial episode renders the originally affected disk atrophic.

Retrobulbar Optic Neuropathy

The retrobulbar portion of the nerve may be affected by most of the diseases that affect the optic disk. The clinical picture is similar except that there is no disk edema acutely, but optic atrophy may follow later. When ON strikes the retrobulbar portion of the nerve, marked visual impairment occurs, but the disk appearance remains normal, because the pathology is posterior to the papilla. Optic papillopathy thus causes impaired vision and an abnormal disk; retrobulbar optic neuropathy causes impaired vision and a normal disk; and papilledema causes an abnormal disk but does not affect vision acutely. An old saw describes these differences aptly: when the patient sees (has normal vision) and the doctor sees (observes disk abnormalities), it is papilledema; when the patient doesn’t see (has impaired vision) and the doctor sees (observes disk abnormalities), it is papillitis; when the patient doesn’t see (has impaired vision) and the doctor doesn’t see (observes no disk abnormality), it is retrobulbar neuritis.

A major difference between retrobulbar neuropathy and papillopathy is the increased incidence of compression as an etiology in the former. Mass lesions of many types, particularly neoplasms, can affect the retrobulbar optic nerve. Common causes include meningiomas of the optic nerve sheath or sphenoid wing, pituitary tumors, and distal carotid aneurysms. The possibility of compression always figures prominently in the differential diagnosis of patients with optic neuropathy. Insidious visual loss producing decreased acuity; impaired color perception; and central, cecocentral, or arcuate scotoma is typical. Compressive neuropathies may evolve more acutely in patients with metastatic lesions, particularly lymphoma. The optic neuropathy in low-pressure glaucoma may simulate the picture of compression.

Distal (Prechiasmal) Optic Neuropathy

Disorders that affect the distal portion of the optic nerve near its junction with the chiasm are similar to other retrobulbar optic neuropathies except that involvement of the Wilbrand’s knee fibers may produce a junctional scotoma, which is highly localizing when present (Figure 13.19C). For this reason, it is important to pay particular attention to the temporal field of the opposite eye when examining a patient with optic neuropathy. The most common cause is pituitary tumor.

Chiasmal Lesions

Pituitary tumors, craniopharyngiomas, meningiomas, gliomas, and carotid aneurysms are the lesions that commonly involve the chiasm. Uncommon causes include demyelination, ischemia, radionecrosis, and a host of other conditions. Because the chiasm lies about a centimeter above the diaphragma sella, visual system involvement indicates suprasellar extension; chiasmatic mass effect is a late, not an early, manifestation of a pituitary tumor (Figure 13.6). Involvement of macular fibers may produce bitemporal scotomas. Chiasmal lesions rarely produce textbook bitemporal hemianopias. There is often a combination of chiasm and optic nerve or optic tract defects depending on whether the chiasm is prefixed, postfixed, or in normal position and the particular attributes of the mass and its force vectors (Figure 13.7). Generally, the defects are binocular and usually heteronymous. The deficit may develop so slowly as to pass unnoticed by the patient. Acuity, color vision, and pupillary function are not affected unless there is optic nerve involvement. Although binasal hemianopias can occur from chiasmal disease, optic neuropathy, glaucoma, and congenital anomalies are more common causes.

Retrochiasmal Lesions

Retrochiasmal lesions produce contralateral homonymous VF defects that respect the vertical meridian. Except for optic tract lesions, they do not cause any deficit of visual acuity, color perception, pupillary reactions, or disk appearance.

Optic tract and LGB lesions occur rarely, perhaps because of generous collateral blood supply; they are characterized by incongruous homonymous hemianopias that split the macula. Optic tract lesions may be accompanied by a mild APD in the contralateral eye because of a greater percentage of crossed pupillomotor fibers (see Chapter 14). Tract lesions may also result in bow-tie pattern disk pallor in the contralateral eye (Box 13.7) and more generalized pallor in the ipsilateral eye. Visual acuity remains normal. Etiologies of optic tract lesions include masses (e.g., meningioma, glioma, craniopharyngioma), aneurysms, AVMs, demyelinating disease, and trauma. Rarely, an APD can be seen with lesions elsewhere in the retrochiasmal pathways and even in the midbrain. Behr’s pupil refers to a slightly dilated pupil because of an optic tract lesion, usually associated with a contralateral hemiparesis.

LGB lesions are rare and usually because of vascular disease. They cause a contralateral homonymous hemianopia that is somewhat incongruous, occasionally with a wedge-shaped or hour-glass pattern along the horizontal meridian pointing to fixation (sectoranopia or keyhole defect) and splits the macula. The unusual pattern is due to the organization of the LGB and to its dual blood supply. Etiologies of an LGB lesion include ischemia, neoplasm, AVM, demyelinating disease, and trauma.

In geniculocalcarine pathway (optic radiation) lesions, temporal lobe pathology typically produces contralateral superior quadrantanopias, or homonymous hemianopia, worse in the upper quadrants; and parietal lobe processes contralateral inferior quadrantanopias, or homonymous hemianopia, worse in the lower quadrants (Figure 13.19). The more posterior the lesion, the more congruous the defect. Parietal lesions are associated with asymmetric OKN responses. Parietal lobe lesions may be accompanied by other evidence of parietal lobe dysfunction, such as cortical sensory loss, aphasia, apraxia, agnosia, anosognosia, and hemispatial neglect (Chapters 9, 10, and 35).

In the occipital lobe, the upper retinal fibers (lower VF) synapse on the upper bank, and the lower retinal fibers synapse on the lower bank of the calcarine cortex, which is separated by the calcarine fissure (Figure 13.12). The macular representation is massive, taking up the occipital pole and about 40% to 50% of the contiguous cortex. Occipital lobe lesions cause contralateral homonymous hemianopias that are highly congruous, tend to spare the macula, and do not affect OKN responses. Macular sparing is thought to be due in part to middle cerebral artery collaterals that help to preserve macular function despite a posterior cerebral artery territory infarct. Conversely, the occipital pole is an area of border zone perfusion between the middle and posterior cerebral arteries, and hypotensive watershed infarctions may cause contralateral homonymous paracentral scotomas because of ischemia limited to the macular cortex (Figure 13.19D). Bilateral occipital lobe lesions causing bilateral hemianopias may cause decreased visual acuity. Bilateral occipital infarcts with macular sparing may leave only constricted tunnels of central vision, as though looking through pipes. Although acuity may be normal, the functional visual impairment is extreme because of the constricted peripheral vision, analogous to end-stage retinitis pigmentosa. Occipital lobe lesions may spare the monocular temporal crescent if the damage does not involve the anterior part of the cortex. Conversely, small far anteriorly placed lesions may involve only the temporal crescent in the contralateral eye (half [quarter might be more appropriate] moon or temporal crescent syndrome). Preservation of the temporal crescent results in strikingly incongruous fields. Preservation of the temporal crescent has been called an “endangered” finding because it requires the now seldom used kinetic (Goldmann) perimetry; the currently used static perimetric techniques that concentrate on the central 30 degrees of the VF tend to miss this phenomenon (Figure 13.31).

FIGURE 13.31 Loss of the temporal crescent in a patient with an infarct in the right anterior occipital lobe. Kinetic perimetry demonstrates a full peripheral field in the right eye (right), but there is loss of the far temporal field (the temporal crescent) in the left eye (left). (Reprinted from Miller NR, Biousse V, Newman NJ, et al. Walsh and Hoyt’s Clinical Neuro-Ophthalmology: The Essentials. 3rd ed. Philadelphia: Wolters Kluwer, 2016, with permission.)

Bilateral occipital lesions may also cause some dramatic defects of cortical function in addition to the visual loss. Anton’s syndrome is cortical blindness because of bilateral homonymous hemianopias, with extreme visual impairment in which the patient is unaware of, and denies the existence of, the deficit. Anton’s syndrome and related disorders are discussed in Chapter 10.

Most occipital lesions are vascular. Many anterior temporal lobe lesions are neoplastic. Parietal lesions may be either. The greater likelihood of tumor in the parietal lobe gives rise to Cogan’s rule regarding OKNs (see Chapter 14). Trauma, vascular malformations, abscesses, demyelinating disease, metastases, and other pathologic processes can occur in any location.

Other Abnormalities of the Ocular Fundus

Other abnormalities of the fundus are also important to detect in neurologic patients. The fundus may reveal evidence of hypertensive retinopathy in the patient with stroke, especially in the lacunar syndromes. In the patient with hypertensive encephalopathy, there may be spasm of retinal arterioles. Retinal emboli may be seen in the patient with possible cerebrovascular disease. In the patient with acute severe headache, the finding of subhyaloid (preretinal) hemorrhage is pathognomonic for subarachnoid hemorrhage (Figure 13.32). The presence of a cherry red spot indicates a condition such as gangliosidosis, lipid storage disease or mucopolysaccharidosis in the younger patient (e.g., Tay-Sachs disease), or a central retinal artery occlusion in the older patient (Figure 13.33). In storage diseases, the cherry red spot is seen because of the accumulation of abnormal material within the cell layers of the retina. Because of the relative transparency of the macula, the underlying choroid is visible. In central retinal artery occlusion, the preservation of blood supply to the macula from the choroidal circulation makes it stand out against the retina made pale by ischemia. Pigmentary retinopathy is seen in such conditions as Kearns-Sayre syndrome and other mitochondriopathies.

FIGURE 13.32 Subhyaloid hemorrhage in a patient with subarachnoid hemorrhage (Terson’s syndrome). The hemorrhage occurs between the posterior layer of the vitreous and the retina, is globular, and often forms a meniscus.

FIGURE 13.33 Cherry red spot in a patient with a lipid storage retinopathy.

Video Links

Video Link 13.1. Afferent pupillary defect. https://collections.lib.utah.edu/details?id=180307&q=sort_type_t%3A%2AMovingImage%2A+AND+afferent+pupillary+defect&fd=type_t%2Ctitle_t%2Cdescription_t%2Csubject_t%2Ccollection_t&rows=50&sort=sort_title_t+asc&facet_setname_s=ehsl_novel_%2A

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